In the fabrication of metal objects, one of the known process steps is the cold working of metals. Cold working involves the physical deformation of a metal article (e.g., a blank, slug, or preform) at a temperature well below the recrystallization temperature of the metal, and preferably within about 200.degree. F. of room temperature. Typical cold working processes include cold extrusion, cold heading, and wire and tube pulling deformation operations.
In order to deform a metal article, all cold working processes involve contacting the metal article with at least one die to work the metal article. When a metal is passed through a die during cold working, the friction between the metal being worked and the die causes the generation of heat. The heat caused by work hardening during deformation also increases the temperature of the die surface and the metal article. The generated heat, in turn, can have detrimental effects on both the dies and on the metal articles. For example, the die can wear rapidly due to the generated heat. Also, the heat can cause surface impurities on the surface of the metal article.
Accordingly, it is desirable to reduce the friction by applying a lubricant to the metal surface and/or the die in order to avoid or minimize the generation of heat during cold working. In order for a lubricant to remain in contact with the metal during one or more cold working drawings, a coating (i.e., a lubricant carrier) should first be adhered to the metal surface.
Numerous zinc phosphate coatings have been proposed in order to achieve this function as a lubricant carrier. It is well known to form metal phosphate coatings by the application of an aqueous acidic phosphatizing solution to metal surfaces. The surface of the metal reacts with the solution to form an integral layer of substantially insoluble crystalline phosphate on the surface of the metal. This crystalline layer increases the ability of the metal to retain a uniform film of lubricant over the entire surface. This lubricant reduces metal to metal contact in cold forming operations. In addition to serving as a lubricant carrier in preparation for cold working, some zinc phosphate coatings can improve corrosion resistance of the metal surface and improve the adhesion of paint to the metal surface.
For example, U.S. Pat. No. 5,588,989 to Vonk et al. discloses the use of an oxime accelerator in a zinc phosphate bath which may also contain fluoride ions, nitrate ions, and ions of various metals, such as nickel, cobalt, calcium, magnesium, manganese, and iron.
In addition, U.S. Pat. No. to 5,603,818 to Brent et al. is directed to a process for treating metal parts to provide rust-inhibiting coatings in at least five discrete steps. The first step in this process is the use of an aqueous phosphatizing solution that is used to deposit a phosphate coating on a metal surface. Various inorganic phosphate solutions are suggested, including zinc phosphate. Chlorate ions are disclosed as an additive to the phosphate solution.
Despite advances in both the composition of the phosphate coating solution and the phosphatizing process, there is a continued demand for still further improvements in the compositions and processes to provide more control over the process and to reduce the time needed to achieve a high quality coating of sufficient coating weight.
After cold working or other fabrication methods, it is desirable to deposit a coating or "pretreatment" on a metal surface in order to improve the adhesion of a subsequently applied decorative coating to the metal surface and to improve corrosion resistance. Before chromium was recognized as environmentally hazardous, compositions containing chromium were used as such pretreatments to improve the corrosion resistance and paint adhesion of metals. Chromium chromate and chromium phosphate were two compounds typically used as coatings for metal. Recent developments led to the use of chrome-free pretreatments, which typically include a polymeric resin and a fluo or oxy-fluo form of group IV-B elements of the periodic table (e.g., titanium, zirconium, and hafnium). These chrome-free pretreatments provide surface coatings whose compositions are based on the products of the interaction between the metallic substrate and the pretreatments.
It is a goal of the present invention to provide a method and composition which farther reduces friction during cold working, can be deposited in a shorter time, improves the corrosion resistance of the metal, and improves the paint adhesion of metal surfaces.